1. Field of the Invention
The present invention relates to a gradient index (GRIN) lens system in which two lenses are employed having a refractive index distribution from the optical axis along the height (i.e. radial refractive index distribution).
2. Description of the Prior Art
As a radial lens system having a refractive index distribution in a direction vertical to the optical axis and an axial lens system having a refractive index distribution along the optical axis, lens systems of comparatively small size such as objective lenses for optical discs have already been put into practical use. Lens systems of large size, however, have not yet been put into practical use because of the problem of the cost of manufacture.
Accordingly, radial and axial lens systems have hardly ever been used as optical systems for use in copying apparatuses which should be of large size and reading lens systems for use in image scanners. However, GRIN lens systems, which can be made compact by reducing the number of lens elements, are highly promising in the future. The following are some of the GRIN lens systems which have been proposed so far: (1) "Design of a Gradient-index Photographic Objective", Applied Optics Vol. 21, No. 6 shows an example of a design of a two-element photographic lens system; (2) U.S. Pat. No. 4,974,947 discloses two-element and three-element axial GRIN lens systems; (3) Japanese Laid-open Patent Application No. H2-285312 discloses a video and a photographic lens system in which at least one radial GRIN lens is used; and (4) "Design of an Optical System in Which a Radial GRIN Lens Is Used" (1991), Lectures at the 16th optics symposium, pages 39 to 40. All of them are photographic lens systems.
The lens system of (1) presents the problems of high cost and inferior processing capability since first and second lens elements thereof are very thick. Further, correction of chromatic aberration is not considered. The lens system of (2) is a wide angle lens system in which an axial GRIN lens is used having a refractive index distribution where the refractive index varies along the optical axis. Only data on a mono color are shown. In the lens system of (3), a radial GRIN lens is used mainly for correcting chromatic aberration. GRIN lens is applied to only one lens element of the photographic lens system including a plurality of lens elements. In the lecture of (4), achromatizing conditions required when the total power which is the sum of the powers of lens configuration and lens material is negative are obtained. As an application thereof, a method to obtain achromatizing conditions required when the total power is positive is shown. However, the example of the design is intended to reduce the total length of a zoom lens system.
On the other hand, for the reading lens systems for use in image scanners, the condition of chromatic aberration is strict compared to the photographic lens system. Accordingly, four to six lens elements are required when the lens system is the conventional spherical lens system of uniform configuration and material. Similarly, in the conventional lens systems for use in copying apparatuses having uniform refractive indices, at least three lens elements are required.
For the copying apparatuses and image scanners, reduction of size is strongly demanded, and accordingly, for the lens systems for use in those apparatuses, reduction of the number of lens elements and reduction of size are also strongly demanded.